1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to optical amplification producing a wider total gain bandwidth.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables can reliably transport optical signals over long distances. However, over a long distance, the optical signals attenuate in the optical fiber due to Rayleigh scattering. The attenuation may be recovered using optical amplifiers, such as discrete amplifiers or distributed amplifiers. Distributed amplifiers use the transmission fiber carrying the optical signals as a gain medium. Discrete amplifiers do not use the transmission fiber as a gain medium, but use another type of fiber or component as the gain medium.
One type of transmission fiber currently used in fiber optic networks is a silicate-based optical fiber. The low attenuation region of a silicate-based optical fiber that can be used for data transmission is about 300 nm (1300 nm to 1600 nm). Unfortunately, the low attenuation region may not be fully utilized for data transmission because current optical amplifiers can only recover optical signals within a narrow gain band.
Many discrete amplifiers provide a gain bandwidth of 30 nm or less. For instance, a C-band EDFA provides about a 30 nm gain bandwidth in the C-band (about 1530 nm to 1560 nm). To achieve a wider gain bandwidth, multiple discrete amplifiers may be used simultaneously. For instance, the C-band EDFA may be used to amplify wavelengths in the C-band simultaneously as another EDFA amplifies wavelengths in the L-band (about 1565 nm to 1620 nm). Simultaneous use of these two amplifiers can provide a gain bandwidth of over 80 nm. A Fluoride-based Thulium-doped fiber amplifier (F-TDFA) may be used to amplify wavelengths in the S-band (about 1460 nm to 1530 nm). Simultaneous use of these three amplifiers can provide a gain bandwidth of over 100 nm. Unfortunately, simultaneous use of these amplifiers can be complex and expensive.
Raman amplification may also be used to get wider gain bandwidths. If a Raman fiber laser pumps a Germanium-doped Silicate fiber at a given wavelength, the pumping may generate a 3 dB gain bandwidth of about 30 nm. To obtain wider gain bandwidths, Wavelength Division Multiplexing (WDM) pumping may be used to pump the optical fiber at multiple wavelengths. The WDM pumping can provide multiple 3 dB gain bands having a bandwidth of about 30 nm, where each gain band corresponds to each wavelength being pumped. However, the total gain bandwidth is limited to less than 100 nm. In Raman amplification, the gain band is at a 100 nm longer wavelength region than the pumping wavelength. The longest wavelengths being pumped can overlap the gain band that corresponds with the shortest wavelength being pumped. Because there is a 100 nm gap between the pumping wavelength and the gain band in Raman amplification, the total gain bandwidth for WDM pumping is limited to less than about 100 nm.
To overcome this limitation, other materials may be used in the optical fiber. If a Raman fiber laser pumps a Phosphate-doped Silicate fiber or Phosphate-Germanium co-doped Silicate fiber at a given wavelength, the pumping generates a gain band at the same 100 nm longer wavelength region as in a Germanium-doped fiber. The gain band at the 100 nm longer wavelength region has a bandwidth of about 30 nm (3 dB). The pumping of the Phosphate-doped fiber or Phosphate-Germanium co-doped Silicate fiber also generates a second gain band at a 250 nm longer wavelength region. Unfortunately, the gain band in the 250 nm longer wavelength region only has a bandwidth of about 9 nm (3 dB). The narrow 9 nm gain bandwidth in the 250 nm longer wavelength region limits the advantages of WDM pumping, as WDM pumping of the Phosphate-doped fiber generates a non-continuous gain bandwidth in the 250 nm longer wavelength region.